The present invention relates to wireless telecommunications systems which use spread-spectrum methods.
Background: Spread-Spectrum Methods
One of the most important tools in telecommunications is spread spectrum methods. For example, in a direct-sequence spread-spectrum ("DS-SS") transmission, the signal is modulated by a code word which is known to both the transmitter and receiver. (The code word is merely a long pseudo-random bit sequence, that is, a sequence of bits which appears random but is determined by the input to a generator and is, therefore, reproducible. The sequence is generated identically, at both the transmitter and receiver, by custom hardware.) At the receiving end, digital filtering methods can be used to selectively recognize only the signals which are encoded with the expected pseudo- random bit sequence. Since the code word is used to separate signals which share the same spectrum space, these methods are also known as CDMA (code-division-multiple-access). To distinguish between the bits of the code word and the bits of data, the bits of the code word are referred to as "chips." The chip rate is usually much faster than the bit rate.
The term "spread spectrum" is also used to refer to two other techniques: "frequency-hopping" systems, in which the transmitter frequency changes in some way which the receiver can predict; and "chirp" modulation or Pulse-FM in which a carrier is swept over a wide band during a given pulse interval. Frequency-hopping systems are less important for civilian telecommunications.
CDMA methods are commonly used in cell phone systems. In such a system, adjacent base stations must have different spreading sequences (long pseudo-noise or "PN" codes), and the mobile unit must be able to lock onto the correct long code (spreading sequence) for each base station it may interface to. The mobile unit will already know the set of possible long codes which it may encounter, but will not know a priori which long code it will encounter when switched on. In most systems the mobile unit will also not know what the received long code offset is, that is, the timing of the transmission of the long code is not known. However, it is highly desirable for the mobile unit to acquire the received long code quickly. This is an essential step in the handoff from one base station to another. Base station handoff is particularly a problem as the number of base stations becomes larger.
Thus the acquisition of the long code during initial acquisition or handoff is a critical bottleneck. A technique for accelerating this has been proposed in the NTT DoCoMo ("Nippon Telegraph & Telephone Mobile Communications Network, Inc.") System. In this system, each base station transmits a signal on a "pilot" (or "perch") channel which helps the mobile system to acquire the correct long code for that base station.
The DoCoMo System
The DoCoMo System introduced some features to make the PN acquisition of the long code simpler. In the DoCoMo system, PN acquisition is accomplished by intermittently broadcasting a symbol which is not encoded by the base station's long code.
Thus, a mobile receiver can simply search for the particular short code ("SC0") which is shared by every base station. When the mobile unit finds this short code, SC0, it can also look to see which of the short codes SCk is being broadcast synchronously with the SC0 code. The SCk code will show which group of transmitters the base station belongs to. The receiver then uses this information to shorten its search through the complete set of codes. Once this information has been acquired, there are still two ambiguities which must be resolved: the receiver must still identify which long code, within the reduced group of possible long codes, is being broadcast; and the receiver must still determine the phase of the long code.
That is, within the timing architecture of the DoCoMo system, a long-code-masked symbol is broadcast once in every ten symbols. Since there are 160 symbols in the complete long code, the long-code-masked symbols will be broadcast 16 times before the long code has repeated once. Thus in the DoCoMo system the receiver has to discover which of the 16 repetitions of the short code (within the long code period) has been detected. (Since it is not known which long code is being used, it is necessary to check all possible phases of the reduced set of possible long codes.) See Higuchi et al., "Fast cell search algorithm in DS-CDMA mobile radio using long spreading codes," 1997 IEEE 47th Vehicular Technology Conference vol.3 pp.1430-4, which is hereby incorporated by reference.
Spread-Spectrum Telephony with Accelerated Code Acquisition
The present application discloses an improved mobile communications architecture, in which each base station broadcasts not only data which has been spread by that station's long code word, but also (intermittently) code identification data which has not been spread. The code identification data is a block code which includes multiple symbols, so that multiple intermittent transmissions are required to complete the transmission of the code identification data. This transmission lets the mobile station shorten the search for the base station's long code word in two ways: the code identification data gives at least some information about the long code itself; and the phase of the block code gives at least some information about the phase of the long code word.
This advantageously results in a system where the amount of searching which a mobile unit must do to acquire the correct code for a new base station is greatly reduced. This results in the further advantage of faster acquisition of a new base station during hand off.